1. Field of Invention
This invention relates to a hexacene derivative that can serve as a precursor of hexacene, a method for forming hexacene from the hexacene precursor, a method for forming a hexacene crystal from thus formed hexacene, a process for making an organic semiconductor device that utilizes the method for forming a hexacene crystal, and an organic semiconductor device made through the process.
2. Description of Related Art
Acenes are a class of aromatic hydrocarbons composed of linearly fused benzene rings. Functional acenes are of contemporary interests both in theoretical aspect and as a new type of organic materials. One of the most attractive features of acenes is their exceptionally narrow HOMO-LUMO band gap, which leads to the highest conductivity than any other kinds of organic compounds. The hole mobility in single crystals of acenes, measured in organic field-effect transistor (OFET) across gold electrodes on top of SiO2, increases with the number of aromatic rings, e.g., anthracene (μFET, 0.02 cm2 V−1 s−1)<tetracene (μFET, 0.4 cm2 V−1 s−1)<pentacene (μFET, 1.4 cm2 V−1 s−1).
Although larger acenes have great potential in a wide range of applications, their utilities are limited severely by both the low solubility and low stability in solutions. Hexacene, for example, has been reported more than 70 years ago, yet its property has never been unambiguously described until recently. The reason was mainly due to its tedious synthesis and low solubility.
To increase the solubility, a series of peri-functionalized derivatives containing silylethynyl and organothio substituents have been synthesized. These types of derivatives, including heptacene and nonacene, have indeed showed higher stability and solubility in solutions. For non-substituted hexacene, a recent synthesis has been achieved through a photo-induced expulsion of CO molecules from a diketone precursor, while the product was collected by matrix isolation, as described in Mondal, R. et al., “Revisiting the stability of hexacenes”, Org. Lett. 9, 2505-2508 (2007). Under room temperature in the polymer matrix, the product can be kept for more than 12 hours. However, hexacene crystals could not be obtained by this method.
A similar matrix isolation has also been conducted recently on the preparation of non-substituted nonacene. It has been established that the photo-expulsion of CO through diketone precursors proceeded through biradical intermediates. In the case of pentacene, the yield was only 74% (τT=48.48±0.15 μs) from its diketone precursor.